Ring magnets of the kind as stated above are being applied e.g. as motor magnets, with the basic body holding the magnet being fixed to a motor shaft by means of press fit. When designing the basic body and the magnet, it should be taken into consideration that--if at all possible--no tensions should be transmitted to the ring magnet due to press fitting the basic body onto the shaft in order to avoid its tearing off the basic body. A known measure to avoid a direct transmission of radial tensions from the shaft to the ring magnet is to lay out the inside diameter of the basic body smaller than the one of the magnet. Further, the basic body is laid out in such a way due to its material, e.g. plastic material, and its design, e.g. axially running openings, that a transmission of tensions via the basic body to the ring magnet can be avoided to a large extent.
For injection-moulding reasons, the outside diameter of the basic body corresponds to the one of the ring magnet in order that the magnet can be injected via a core to the basic body in a mould corresponding to the inside diameter.
The application of the ring magnet as a motor magnet involves the requirement to locate the ring magnet at the basic body in such a way that safety against torsion and axial shifting is guaranteed. For this purpose, frictional and/or positive connections are required, as glued connections do not guarantee a permanent safe fixing. In this respect, known designs are insufficient and not adequate at all.
From DE-PS 687 560 a rotor is known the ring-shaped magnet body of which is mounted onto a steel bush with which it is joined by welding at conically designed edges. The steel bush with the magnet body are secured against torsion on a shaft by means of a key.
JP 61-35501(A) in Patents Abstracts of Japan E-417 of Jul. 5, 1986, Vol. 10/No. 192, shows a cylinder-shaped basic body with grooves running in vertical direction which is axially inserted into the boring of a ring magnet. Before and after inserting, the magnet is coated for the purpose of increasing its mechanical strength.
In DE-GM 1 804 397, a ring magnet is described which is located on a ring-shaped basic body held by keys. This is achieved by using either a ring-shaped key which is indented into the appropriate recess of the basic body deforming it in such a way that the basic body is pressed to a shaft as well as to the magnet. In another version, the basic body consists of two intermeshing ring-shaped keys which tightly connect the magnet with the shaft by means of mutual pressing.
DE-AS 1 043 540 and DE-PS 965 464 show ring magnets which are fixed by means of glueing onto a bush extending along a part of the axial groove of the magnet. The glued connection does not guarantee a permanent safe fixing of the magnet to the bush.
Therefore, the invention is based on the task to create a positive connection between the ring magnet and the basic body which guarantees in particular an improved safety against torsion of the ring magnet on the basic body by injecting the ring magnet in a simple way to the basic body.
The task is solved as per the invention in such a way that at a plastic-bonded ring magnet with a ring-shaped basic body of the kind as described in the beginning, the basic body shows at least one axially directed channel-shaped recess which is filled with magnetic material.
On the one hand the axially continuous recess, which is provided in the basic body and into which the ring magnet extends with an appropriately shaped part, offers a sufficient safety against torsion of the magnet on the basic body. On the other hand, the magnetic material for manufacturing the magnet, which is fluid in heated state, can be injected through the recess into the hollow space of the mould, a fact which proved to simplify the injection-moulding manufacture of the ring magnet.
The recess may partly or fully be filled with magnetic material. At the end opposite to the hollow space of the mould, the recess advisably shows an area which serves as an injecting aid and is not filled with magnetic material after injection-moulding. This area may show a widening which is adjusted to the injection nozzle.
With an advantageous further development of the invention, the recess shows a tapering of the cross section towards the ring magnet. Due to the part of the ring magnet which is located in the tapering of the cross section and adjacent to the vicinal front of the basic body, safety against torsion as well as axial shifting of the magnet on the basic body is guaranteed.
In a preferred further development of the invention, the recess is opened towards the circumferential surface of the basic body. In this case it forms a continuous channel extending axially along the circumference. The part of the ring magnet extending into this channel, shows a surface in accordance with the outside diameter of the basic body which is subject to the injection-moulding form.
With this version, the above mentioned tapering of the cross section may be laid out in such a way that the wall areas of the recess adjoining the circumferential surface of the basic body run up to each other towards the ring magnet.
Further or on the other hand it may be provided that the radial inside wall area of the opening which is formed at the outside of the basic body, inclines in axial direction towards the circumferential surface. The conicality in radial direction, brings about a safety against shifting of the magnet on the basic body, too.
The angles of the lateral wall areas running up to each other and/or the radially inclined wall area of the recess are chosen in such a way that with the cooling of the magnetic material after injection-moulding no contraction strains occur which may lead to the magnet tearing off in the area extending into the recess.
In a preferred version, the recess at the outside of the basic body shows a essentially U-shaped cross section which is narrowed by foot-shaped areas at the circumference surface of the basic body. This narrowing guarantees a safety of the magnetic material extending into the recess against the centrifugal forces occurring with high rotational speed of the ring magnet.
For technical reasons of injection-moulding, the number of recesses and their layout can be determined by the number of poles and/or their layout. A two-pole magnet for instance would preferrably be provided with two diametrally opposite recesses.
In a suitable further development of the invention, the basic body shows at its front facing the magnet a essentially cylindrical projection representing further safety against torsion. The safety against torsion preferrably consists of a flattening which suitably is facing the recess in order that it can cover a sufficient cross section surface.
It is advisable not to use the projection for press fitting of the basic body onto the shaft. For this reason, the inside diameter of the projection is slightly bigger than the one of the remaining basic body. It can correspond to the inside diameter of the magnet so that during its injection-moulding manufacture the core extends into the cylinder-shaped projection.
Advantageously, the inside diameter of the projection is continuing slightly into the remaining basic body in order that it shows an accordingly larger inside diameter at its end facing the ring magnet. By means of this measure, the transmission of axially oriented tensions via the basic body, which is press-fitted onto the shaft, to the ring magnet is avoided to a large extent.
A preferred version of the invention is further specified as per the following drawing.